1. Field of the Invention
The present invention relates generally to marine seismic exploration and, more particularly, to a technique for achieving multiple attenuation on marine seismic data.
2. Description of the Related Art
In marine seismic exploration, a seismic survey ship is equipped with an energy source and a receiver for taking seismic profiles of an underwater land configuration. The act of taking profiles is often referred to as "shooting" because explosive devices have been commonly used for many years as energy sources. The energy source is designed to produce compressional waves, commonly referred to as "primary" waves, that propagate through the water and into the underwater land formation. As the compressional waves propagate through the land formation, they strike interfaces between the formations, commonly referred to as "strata," and reflect back through the earth and water to the receiver. The receiver typically converts the reflected waves into electrical signals which are then processed into an image that provides information about the structure of the subterranean formation.
Presently, one of the most common energy sources is an airgun that discharges air under very high pressure into the water. The discharged air forms a pulse which contains frequencies within the seismic range.
The receivers in marine applications are typically referred to as hydrophones. The hydrophones convert pressure waves into electrical signals which are used for analog or digital processing. Most commonly, hydrophones include a piezoelectric element for converting the pressure waves into electrical signals. The hydrophones are mounted on a long streamer which is towed behind the survey ship at a depth of about 30 ft. It is not uncommon for the streamer to be several miles long and to carry receivers every few feet in a regularly spaced pattern.
As previously mentioned, each time the energy source imparts a seismic pulse into the water, the compressional waves propagate through the land formation, strike strata, and reflect back through the earth and water to the receivers. Each receiver detects the reflected wave, and delivers an electrical signal to a recording device aboard the survey ship. Each recorded signal from a receiver is commonly referred to as a "trace." Thus, for each seismic pulse generated, many traces may be recorded.
The wave that is reflected off of the strata and detected by the receivers is commonly referred to as a "primary reflection." Unfortunately, the receivers detect pressure waves other than the primary reflection. For instance, a problem encountered in marine seismic surveying is that of water column reverberation. This problem arises as a result of the inherent reflectivity of the water surface and the water bottom. After the primary reflection travels upwardly past the receiver, the wave continues upward to the water's surface. The primary reflection reflects off of the air-water interface and begins to travel downwardly toward the water bottom where it is again reflected. Thus, this multiply reflected wave, often referred to as a water bottom multiple, travels past the receivers again. The receivers detect this reflection and the reflection is recorded on the traces. Depending upon the nature of the earth's material at the water bottom, the multiple may itself be reflected again, and give rise to a series of one or more subsequent multiple reflections.
This reverberation of the seismic wave field in the water obscures seismic data, amplifying certain frequencies and attenuating others, thereby making it difficult to analyze the underlying earth formations. When the earth material at the water bottom is particularly hard, most of the acoustic energy generated by the seismic source can become trapped in the water column. As a result, the multiple energy tends to cover the weaker primary seismic reflection signals sought for study.
In an effort to isolate the data produced by the primary reflections from the data produced by reverberation and other noise sources, engineers model the undesirable data produced by multiple reflections for each data trace. Theoretically, the model of the undesirable data can be subtracted from the recorded data trace to yield a clean data trace that contains only the data produced by the primary wave.
However, the current modeling techniques cannot achieve this theoretical result. In one technique, an initial estimate of the undesirable data is formed using a wave field extrapolation technique. See J. Claerbout, Imaging the Earth's Interior (1985). Wave field extrapolation is often used to build the model trace, but the model so constructed may be in error for the following reasons:
(1) the velocity of sound in the multiple generating layer is not accurately known;
(2) the thickness o the multiple-reflection generating layer is not accurately known;
(3) the magnitude of the reflection coefficients at the reflection boundaries may not be well known; and
(4) the reflection boundaries may not be single interfaces.
These problems result in a distorted model. Therefore, the model trace is processed to provide a closer approximation to the recorded data trace. Conventionally, this has been achieved using cross-equalization of the model to the real data. Cross-equalization involves the determination of a filter, which, when applied, will make one seismic trace look like another. This method works well, but exhibits an intrinsic problem. If multiple-reflection energy lies directly on top of primary energy on the recorded data trace, i.e., the receiver detected the multiple wave and the primary reflection at the same time, then the cross-equalization filter will force the model of the multiple-reflection energy to look like the primary and multiple energy on the recorded data trace. Thus, cross-equalization filters treat desirable primary reflection data as undesirable multiple-reflection data, and subsequent subtraction of the cross-equalized model from the recorded data trace will attenuate the primary energy.
The present invention is directed to overcoming, or at least minimizing, one or more of the problems set forth above.